Re: [PATCH] riscv: Format Rd of FMV.W.X with NoN-boxing

[Ian Jiang <ianjiang.ict@gmail.com>]

Richard Henderson <richard.henderson@linaro.org> 于2020年1月23日周四 上午11:07写道：
>
> On 1/22/20 2:48 PM, Ian Jiang wrote:
> > Richard Henderson <richard.henderson@linaro.org> 于2020年1月23日周四 上午1:11写道：
> >>
> >> On 1/21/20 11:53 PM, Ian Jiang wrote:
> >>> --
> >>> Ian Jiang
> >>>
> >>> Richard Henderson <richard.henderson@linaro.org> 于2020年1月22日周三 下午4:53写道：
> >>>>
> >>>> On 1/21/20 9:24 PM, Ian Jiang wrote:
> >>>>> For FMV.W.X that moves the lower 32 bits of an integer register to a
> >>>>> floating-point register, Rd should encoded with NoN-boxing scheme.
> >>>>> Note: This applies to RV64 only.
> >>>>>
> >>>>> Signed-off-by: Ian Jiang <ianjiang.ict@gmail.com>
> >>>>> ---
> >>>>>  target/riscv/insn_trans/trans_rvf.inc.c | 1 +
> >>>>>  1 file changed, 1 insertion(+)
> >>>>>
> >>>>> diff --git a/target/riscv/insn_trans/trans_rvf.inc.c b/target/riscv/insn_trans/trans_rvf.inc.c
> >>>>> index 172dbfa919..62b7a36567 100644
> >>>>> --- a/target/riscv/insn_trans/trans_rvf.inc.c
> >>>>> +++ b/target/riscv/insn_trans/trans_rvf.inc.c
> >>>>> @@ -368,6 +368,7 @@ static bool trans_fmv_w_x(DisasContext *ctx, arg_fmv_w_x *a)
> >>>>>
> >>>>>  #if defined(TARGET_RISCV64)
> >>>>>      tcg_gen_mov_i64(cpu_fpr[a->rd], t0);
> >>>>> +    tcg_gen_ori_i64(cpu_fpr[a->rd], cpu_fpr[a->rd], 0xffffffff00000000ULL);
> >>>>>  #else
> >>>>>      tcg_gen_extu_i32_i64(cpu_fpr[a->rd], t0);
> >>>>>  #endif
> >>>>>
> >>>>
> >>>> This doesn't look right.  There's nothing in the spec that says the nan-boxing
> >>>> is restricted to rv64.  NaN-boxing is all about FLEN, not XLEN.
> >>>>
> >>>
> >>> Why the translation of FLW has a NaN-boxing?
> >>>
> >>> file ./target/riscv/insn_trans/trans_rvf.inc.c
> >>>  26 static bool trans_flw(DisasContext *ctx, arg_flw *a)
> >>>  27 {
> >>> ...
> >>>  34     tcg_gen_qemu_ld_i64(cpu_fpr[a->rd], t0, ctx->mem_idx, MO_TEUL);
> >>>  35     /* RISC-V requires NaN-boxing of narrower width floating point values */
> >>>  36     tcg_gen_ori_i64(cpu_fpr[a->rd], cpu_fpr[a->rd], 0xffffffff00000000ULL);
> >>> ...
> >>>  41 }
> >>
> >> Eh?  Obviously because we're loading a 32-bit value into a FLEN=64 bit register.
> > FMV.W.X is the same with FLW at this point that filling a 64 bits
> > float register based on 32 bits value.
> > Besides, the RISCV simulator Spike makes NaN-boxing for FLW, FMV.W.W,
> > FADD.S, FSUB.S, and others.
> > It might be better that they have a coincident behavior? I am not sure
> > about this and just want a discussion.
>
> So, I wonder if we're talking past one another.
>
> In reply to the patch at the beginning of this thread, I was suggesting that
> the correct solution is
>
>  #if defined(TARGET_RISCV64)
>      tcg_gen_mov_i64(cpu_fpr[a->rd], t0);
>  #else
>      tcg_gen_extu_i32_i64(cpu_fpr[a->rd], t0);
>  #endif
> +    tcg_gen_ori_i64(cpu_fpr[a->rd], cpu_fpr[a->rd],
> +                    0xffffffff00000000ULL);
>
> However, after the previous follow-up I wonder if it might be better to split
> out the nan-boxing to a helper:
>
> /*
>  * RISC-V requires NaN-boxing of narrower width floating
>  * point values.  This applies when a 32-bit value is
>  * assigned to a 64-bit FP register.  Thus this does not
>  * apply when the RVD extension is not present.
>  */
> static void gen_nanbox_fpr(DisasContext *ctx, int regno)
> {
>     if (has_ext(ctx, RVD)) {
>         tcg_gen_ori_i64(cpu_fpr[regno], cpu_fpr[regno],
>                         MAKE_64BIT_MASK(32, 32));
>     }
> }
>
> which could elide this when only RVF is enabled, and thus
> the upper 32-bits of the register are inaccessible.
>
> This helper would then be propagated to the existing uses within translate.c
> and insn_trans/trans_rvf.inc.c.
>
> As another cleanup, the ifdef above may be replaced with
>
>     tcg_gen_extu_tl_i64(cpu_fpr[a->rd], t0);
>
> which will handle both RISCV64 and RISCV32.

Good point.
But I am not clear where to call this new helper gen_nanbox_fpr(). Is
there a position that could affect all floating-point instructions? So
that we don't have to modify so many translating functions. Please
give more details.